1. Field of the Invention
The present invention generally relates to an electrophotographic device, and more particularly to a transporting device and method of correcting a shape of a transported sheet.
2. Description of the Related Art
A conventional electrophotographic printing device uses a printing method involving electrostatic force. The basic principles of such a conventional electrophotographic printing device are illustrated in FIG. 8.
In this printing method, a photosensitive body 1 is formed by a cylindrical or a belt-shaped member. A laser (not referenced) or the like is used to form an electrostatic latent image on the photosensitive body 1. A powdered material (e.g., paint) used for printing, generally referred to as a toner 2, is adhered to the electrostatic latent image, and this visible image (e.g., a toner image) is transferred to a sheet 4 serving as a recording medium (e g., cut paper) using a device that uses electrostatic force, generally referred to as a transfer device 3. This results in a printed image
In this case, forming a good toner image on the sheet 4 serving as the recording medium requires adequate contact between the sheet 4 and the photosensitive body 1.
After the toner image is transferred to the sheet 4, the sheet 4 and the photosensitive body 1 must be quickly disengaged. In electrophotographic devices, this separation of the sheet 4 from the photosensitive body 1 is generally also performed using electrostatic force. Generally, the sheet 4 is disengaged by adjusting the strength of a corona discharge from the transfer device 3 based on the relationship between the electrostatic charge of the photosensitive body 1 and the toner 2, and the electrostatic charge of the sheet 4.
This separation method involving electrostatic force in principle can be expected to effectively separate the sheet 4 if there is no shape irregularity of the sheet 4 such as warping. However, if there is shape irregularity (e.g., warping) in the sheet 4, then the separating ability of the electrostatic force will be unbalanced due to the rigidity or the like of the sheet itself. This can lead to the sheet 4 staying wrapped around the photosensitive body 1.
In particular, if the photosensitive body 1 is formed with a cylindrical shape having a curvature, the sheet 4 will tend to stay wrapped around the photosensitive body 1 more frequently with paper shape that is warped in the same direction as the curvature of the photosensitive body 1 (e.g., the surface of the paper 4 on which printing is to be performed is curved concavely relative to the photosensitive body 1). That phenomenon is caused by a force of static electricity which causes the sheet to stick to the photosensitive body in the small area between sheet and the photosensitive body
It is noted that sheet deformation is generally related to heat and humidity.
In electrophotographic devices that perform two-sided printing, the heat applied to fix the toner image to the sheet makes deformation unavoidable. Also, for some types of sheets, the production process thereof will lead to the sheets being formed with deformations such as warping from the start. Thus, the sheets being transported to the image forming module may not necessarily have an ideal flat shape.
In one method used in conventional devices, such a problem is addressed by placing a claw member 5 in contact with the photosensitive body 1 to separate the sheet 4 wrapped around the photosensitive body 1, as shown in FIG. 8. With this method, however, the claw member 5 comes directly into contact with the photosensitive body 1, thereby possibly damaging the photosensitive body 1. This may raise a new problem, because a damaged photosensitive body 1 will have a relatively short service life. Additionally, adhesion of contaminants to the claw member 5 can lead to contaminants becoming adhered to the sheets 4, thereby negatively affecting the print quality.
According to another method, air pressure is used to separate sheets that cannot be separated with electrostatic force. However, this method requires complex devices for performing air suction or compression, thereby leading to increased costs and a less compact design.
U.S. Pat. No. 5,066,984 discloses a device for eliminating sheet warping after printing. FIG. 9 shows a simplified drawing of such a device. In this device, sheet warping is eliminated by passing (e.g., as shown by dotted lines) a sheet through an arcuate gap formed between an arcuate member 6 and a driven roller 7 after the sheet has been printed. Eliminating (or minimizing) warpage requires setting an arcuate gap to 0.6 mm or less (between arcuate member 6 and driven roller 7). Therefore, if the thickness of the sheet is high (e.g., cardboard), then the sheet may not be able to pass through the arcuate gap.
Alternatively, the warpage elimination may be more than necessary, leading to reverse-warping, thereby leading to problems in sheet transport and the stacking of sheets in the sheet ejection device U.S. Pat. No. 5,066,984 proposes a switchable gate disposed at the entry to the arcuate gap with the passage to the device being switched by selecting sheet thickness.
Thus, in the conventional electrophotographic devices described above, a sheet can become wrapped onto the photosensitive body if the side of the sheet to be printed is warped concavely (e.g., printing is to be performed on a sheet having a printing surface that is curved concavely). Also, using a claw member to solve this problem may lead to new problems including shorter service lives of photosensitive bodies and adhesion of contaminants thereto
Moreover, with the method involving air pressure, the cost and size of the device increases. Furthermore, the device to eliminate warping after printing described above requires switching the path to the device depending on the thickness of the sheet.
In view of the foregoing and other problems, drawbacks, and disadvantages of the conventional methods and structures, an object of the present invention is to provide a method and electrophotographic device which overcome the problems described above.
Another object of the invention is to prevent sheets from becoming wrapped around the photosensitive body by installing a device that eliminates curvature of the sheet that is concave relative to the printing surface, without needing to switch the transport path according to a sheet thickness.
In a first aspect of the invention, an arcuate shaped section (e.g., configuration) is disposed on a transport path used to transport a sheet to an image forming module. The arcuate shape is convex relative to the side of the sheet on which printing is to occur. A roller opposing the arcuate section presses the sheet against the arcuate section with an appropriate pressure when the sheet passes the arcuate section The roller is driven in a manner synchronized with the passage of the sheet. The rotation radius of the roller is roughly identical to the curvature radius of the arcuate section The roller is slidable in a direction perpendicular to that of its rotational axis.
In a second aspect of the invention, a shape-correcting device for sheets, includes a guide member for guiding a sheet along a predetermined transport path, an arcuate section formed on a section of the guide member, a roller positioned opposite the arcuate section, and a driving mechanism for driving the roller
In a third aspect of the invention, a method of shape-correcting sheets, includes transporting a sheet along a predetermined transport path, the predetermined transport path including a path between an arcuate section formed on a section of a sheet guide member and a roller positioned opposite the arcuate section, driving the roller in synchronization with a transport timing of the sheet, and pushing the roller in a direction perpendicular to a roller rotation axis and pressing the sheet against the arcuate section of the sheet guide member.
Further, in electrophotographic devices used to print to two sides of a recording medium, the inventive structure can be installed upstream from the image forming module and downstream from where the transport path meets a return transport path used for two-sided printing.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-288084, filed on Sep. 21, 2001, which is expressly incorporated herein by reference in its entirety.